Release of hydrogen sulfide during microwave pyrolysis of sewage sludge: Effect of operating parameters and mechanism

The effects of sludge characteristics, pyrolysis temperature, heating rate and catalysts on the release of H₂S and mechanism of H₂S formation during sludge pyrolysis were investigated in a microwave heating reactor (MHR). The evolution of sulfur-containing compounds in the pyrolysis chars obtained at temperature range of 400-800°C was characterized by XPS. For a given temperature, the maximum concentration of H₂S appeared at moisture content of 80%. Compared to the influence of heating rate on the H₂S yields, pyrolysis temperature and catalyst played a more significant role on the release of H₂S during microwave pyrolysis process. The H₂S concentration increased with increasing temperature from 400°C to 800°C while decreased with increasing heating rate. Both the Nickel-based catalyst and Dolomite displayed significant desulfurization effect and Ni-based catalyst exhibited the larger desulfurization capability than that of Dolomite. The organic sulfur compounds accounted for about 60% of the total sulfur in the sludge which was the main reason for the formation of H₂S. The mechanism analysis indicated that the cleavage reactions of mercaptan and aromatic-S compounds at temperatures below 600°C and the cracking reaction of sulfate above 700°C respectively were responsible for the H₂S release during sludge pyrolysis.